The present invention relates to an image pickup apparatus. More particularly, the present invention relates to an improvement in a signal output section in the image pickup apparatus such as a digital still camera that outputs charges obtained by photoelectric conversion.
The number of pixels in a digital still camera is becoming even larger in recent years. On the other hand, multiplication of the pixels makes it difficult to perform a monitoring operation (the monitoring operation is normally in 1/15 second to 1/60 second).
In order to cope with such a problem, a conventional digital still camera performs thinning of a matrix in a vertical direction and increases a horizontal transfer frequency such that a frame frequency fits within the above-described monitoring operation of 1/15 second to 1/60 second. However, the thinning in the vertical direction of the matrix, so as to cope with further multiplication of the pixels, impairs a balance between horizontal resolution and vertical resolution, leading to degradation of image quality, while the increase in the horizontal transfer frequency significantly raises power consumption and may also causes a failure such as a transfer trouble.
As measures for those disadvantages, a type of charge-coupled device (CCD) having a plurality of horizontal transfer paths and output amplifiers now attracts attention. The problem on the frame frequency relating to the monitoring operation can be solved by such a type of CCD even when the multiplication of the pixels progresses further.
However, it is difficult to uniform a gain characteristic of each of the output amplifiers in a conventional technology. Hence, a problem occurs that, for example, amounts of output signals become different in every portion of the matrix divided by the output amplifiers into regions even when a photographic object having completely even luminance is photographed.
To solve the problem rendered by the conventional technology, an additional correction circuit may be employed in a digital signal processor (DSP), although this leads to increase in a price of the DSP. Even when the DSP having such a correction circuit capable of compensating the gain characteristics is employed and utilized, an adjustment process for adjusting input/output characteristics of the correction circuit becomes essential in addition to a production process.
Moreover, despite the adjustment of the correction circuit, it is difficult to make the gain characteristic of each of the output amplifiers to the same level completely, and thus a slight disparity of the gain characteristics between the output amplifiers is inevitably generated. In a commercial product which handles a still image such as the digital still camera, however, the slight disparity may lead to a crucial drawback.